Production of coated paper utilizing aqueous coatings containing particulate filler and resins having an affinity for water and utilizing nonequilibrium moisture conditions and shearing forces



3,338,735 mes CONTAINING FFINITY P. O. HAIN PRODUCTION O OATED PAPERUTILIZING AQUEOUS COAT PART LATE FILLER AND RESINS HAVING AN A FOR WATERAND UTILIZING NON-EQUILIBRIUM MOISTURE CONDITIONS AND SHSEGASING FORCESFiled Dec. 9, 1

INVENTOR PAUL Q HA/N BY WOPZZ TTORNEY United States Patent 3,338,735PRODUCTION OF COATED PAPER UTILIZING AQUEOUS COATINGS CONTAINING PARTIC-ULATE FILLER AND RESINS HAVING AN AF- FINITY FOR WATER AND UTILIZINGNON- EQUILIBRIUM MOISTURE CONDITIONS AND SHEARING FORCES Paul O. Hain,Hamilton, Ohio, assignor to US. Plywood- Champion Papers Inc., acorporation of New York Filed Dec. 9, 1965, Ser. No. 512,708 19 Claims.(Cl. 117-64) The present invention relates to the production of coatedpaper particularly adapted for printing and is a continuation-in-part ofmy copending application Ser. No. 182,024, filed Mar. 23, 1962, nowUnited States Patent No. 3,268,354.

In accordance with the teachings of the said prior application, anaqueous coating composition comprising a major proportion of filler,especially mineral filler, and a minor proportion of non-waterabsorptive, film-forming, heat softenable resin particles is applied toa surface of a relatively dry fibrous cellulosic paper substrate and theaqueous coating is exposed to heat to dry the exposed surface of thecoating until this exposed surface is dry to the touch and the surfaceof the substrate underlying the coating is sufficiently Wet with wateras to be permanently pressure deformed. While the coated paper is in thenon-equilibrium moisture condition specified, it is passed through thenip between a pair of turning rolls, at least one of which has a heatedfinishing surface, so that heat, pressure and shearing forces areapplied and a print-receptive surface is formed. When materials areselected as indicated and non-equilibrium moisture conditions obtained,as indicated, the turning rolls per the teachings of said applicationcause the production of a superior levelness in the coated surface ofthe paper and improved distribution of the coating over the surface ofthe paper so that coated paper of superior gloss can be produced usingless coating material than was formerly required and the coating is moreuniformly distributed to provide superior ink-receptivity and improvedresistance to blistering in high speed web offset printing.

While the foregoing description is directed to the usual procedure inwhich the water is supplied by the aqueous coating itself, the saidprior application also includes the rewetting of coatings which havebeen previously applied and dried.

As pointed out in said prior application, soluble filmforming agents,interfere with the process and accordingly, the said application islimited to the non-water absorptive particulate resins. Indeed, andprior to the present invention, the use of as little as 2% of casein,based on the solids content of the coating, would cause the coating tostick to the turning rolls and prevent the successful operation of theprocess. However, and using more sophisticated equipment and control, ithas now been found possible to successfully perform the processing stepsof my prior application using resins having a considerable afiinity forwater and which tend to form colloidal solutions in Water in contrastwith the suspensions which are formed when non water absorptive resinsin particulate form are dispersed in water. Thus, the present inventionenables many of the advantages of my prior application to be achieveddespite the use of proportions of casein, starch, polyvinyl alcohol, orother similar thermoplastic resin having considerable solubility inwater. As will be appreciated, the resins mentioned are commonlyavailable inexpensive resins and the capacity of using them whileretaining many of the advantages of my prior application provides asituation of obvious advantage. Similarly, my prior applicationindicates that the non-water absorptive resins should be film-formingpolymers which could be replaced in part, i.e., up to 50% by weight withrubbery polymers such as copolymers of styrene with butadiene andcopolymers of acrylonitrile with butadiene, with or without styrene.When larger proportions of rubbery polymers were used heretofore, thecoating would again stick to the turning rolls and prevent thesuccessful operation of the process. Again, the present inventionpermits the use of rubbery polymers in any desired proportion withrespect to the total binder.

Apparently, drying factors are critically interrelated and, if properlycontrolled, as will be pointed out hereinafter, it is possible to obtainappropriate conditions of surface mobility and concentration andundersurface moisture plasticization even when the coating includescolloidally soluble resin components or rubbery latex particles topermit superior levelness, more uniform coating distribution andsuperior gloss to be obtained, just as these same factors were enhancedin my prior application using non-water absorptive resins. In general,more heat is needed than was adequate using non-water absorptive resinsin view of the fact that the colloidally soluble resins and the rubberylatices have a greater capacity to retain moisture against evaporationand control of temperature is more critical to avoid undesired adhesionto the rolls.

While the invention primarily contemplates the partial replacement ofthe non-water absorptive resin particles with colloidally soluble resinsor rubbery latex particles, as aforesaid, the invention also includesthe complete replacement of the non-water absorptive resins. Also, andwhereas my prior application contemplates coatings containing from60-90% of finely divided solid filler in admixture with from 1040% ofthe particulate, nonwater absorptive resin, the colloidally solubleresins which are used in the invention are intended to be used insmaller amount. Thus, as little as 3% of soluble resin can be used up toabout 30%, based on the mixture of solid filler and resin. Preferredproportions on the same basis are from 520% by Weight.

It is desired to point out that the greater the solubility of the resinin the Water, the smaller is the maximum proportion thereof which can beused. Thus, larger proportions can be tolerated as the average particlesize of the colloidal solution increases.

It is desired to indicate that the non-equilibrium moisture conditionswhich are required in accordance with the present development arecritically interrelated. Starting with paper of ordinary andequilibrated moisture content and which can be characterized asrelatively dry (frequently in the range of 2% to 13% and more frequentlyin the range of 5% to 10% by weight of water as a result of normalstorage), there is deposited on the paper an aqueous coating typicallycontaining from 5070% by weight of water. As a result of this and underconditions of normal coating application, the surface of the paper whichis being coated becomes saturated with water, though the coating doesnot instantly penetrate the paper.

Heat is then applied to dry the coating. Under normal coatingcircumstance, the amount of water applied is not sufiicient (ifuniformly distributed) to render the paper soggy or wet to the touch. Asheat is applied, the applied water partially drains into the surfacelayer of the coating substrate and some of the surface water isevaporated. The applied heat raises the surface temperature until aplateau is reached at a temperature of about F. and which correspondswith the rapid vaporization of immediately available surface Water underthe conditions of heating employed. In accordance with the invention,heating must be continued until vaporization is sufficient to break thecontinuity of the film of water at the surface. If this minimum removalof water is not exceeded, then the passage of the coated product throughthe nip of the turning or calendering rolls applying heat and pressuredoes not produce the result desired by the invention. First, there is astrong tendency, which is particularly pronounced when the resin isselected for colloidal solubility in aqueous medium, for the coating tobe picked off and to accumulate on the calender rolls. Secondly, whenthe coating is subjected to its brief contact with the calender rolls,the heat and pressure applied thereby tends to crush the paper substrateand to produce non-uniform surface characteristics.

There is also an upper limit to the heat drying of the coating which isreached at a temperature of about 195 P. which indicates a sharpreduction in the availability of Water near the surface of the coating.By the time the surface temperature rises above 195 F., theeffectiveness of the invention is reduced and the benefits of theinvent-ion are not effectively obtained. More specifically, Water vaporplasticization is not obtained and the coverage and gloss of the productis impaired. Preferred surface temperatures are in the range of from170190 F. and heating is desirably rapid and completed in a period ofless than 3.0 seconds, preferably less than 1.5 seconds and mostpreferably in less than 0.8 second.

Accordingly, and in accordance with the present invention, an aqueouscoating composition comprising from 70-97% by weight of solid filler,especially mineral filler, and from 330% by weight of colloidallysoluble or dispersible resin or rubbery latex particles is applied to asurface of a relatively dry fibrous cellulosic paper substrate and theaqueous coating is exposed to an intense source of heat to rapidly drythe exposed surface of the coating, heating being continued until thesurface temperature of the coating is in the range discussedhereinbefore. While the coated paper is in the critical nonequilibriummoisture condition specified, it is passed through the nip between apair of turning rolls, at least one of which has a heating finishingsurface so that heat, pressure and shearing forces are applied and theadvantages of my prior application obtained, e.g., one obtains one ormore of superior levelness, improved distribution, and superior gloss atlower coating Weights.

If the paper sheet is dried beyond the point specified, the improvedleveling and shear distribution of the coating is not obtained. Instead,the results degrade rapidly in the direction of conventional supercalendering. Correspondingly, if drying has not proceeded to the pointwhere the surface temperature is elevated above 160 F., then the coatedfilm is not coherent and strong and tends to stick to the rolls and acockled and uneven surface effect is obtained. Also, the pressureapplied by the turning rolls, instead of leveling a specific portion ofthe paper, tends to merely crush the paper and the full glossing effectis not obtained.

In contrast, and when the water content of the coating is substantiallyconfined to that portion of the substrate immediately underlying thecoating, e.g., adjacent the interface between the coating and thesubstrate, permanent deformation of the substrate is largely confined tothe water-plasticized portion underlying the coating to provide maximumleveling and gloss with minimal overall compression and Without losinguniformity of effect or encountering roll sticking.

To relate the present invention to the usual situation, from 1 to 9pounds per ream of coating on a dry basis is applied to a papersubstrate weighing at least 20 pounds per ream with the coating beingaqueous and having a total solids content of from 35-70% by Weight, morefrequently from 4560% by weight, the balance being essentially water.Within this range of application of coating compositions, there isadequate water within the coating to provide the water-plasticizationneeded in the invention. On the other hand, and if desired, the aqueouscoating compositions can be thinned to include a further proportion ofwater if needed.

To the extent that a previously applied cured coating remainsheat-softenable, so that heat and pressure can render the resin bindermobile, reWetting is permissible as in my prior application.

It is important in the preferred practice of the present invention toregulate the heating operation so that the resin binder in the coatingis concentrated with the bulk of the water removed by drainage andevaporation before the water in the paper substrate migrates deeply intothe interior of the paper. The point is to confine water plasticizationto that portion of the paper substrate which immediately underlies theapplied coating. On this basis, it should be seen that the further Wedepart from equilibrium conditions in which the moisture content of thepaper is uniformly distributed across its thickness, the more we areable to realize the advantages of the invention. When heating iscontinued for an excessive period of time, the surface temperaturebecomes excessive, surface moisture is reduced to an unacceptable leveland the water within the paper substrate does not immediately underliethe applied coating which reduces the unique leveling effect obtained inthe invention. It should also be appreciated that the presence ofdissolved resin thickens the aqueous phase and increases the difiicultyof evaporating water from the coating.

In view of the above considerations, the source of heat is preferablyradiant and directed toward the coated substrate from the wet coatedside thereof. Also, the heat should be sufficiently intense in orderthat the required drying is achieved by an exposure of less than about3. 0 seconds as aforesaid.

In the invention the critical non-equilibrium moisture conditions areidentified by a surface temperature of the coated substrate as itemerges from the heating zone of from about F. to about 195 F., asdiscussed hereinbefore. While the defined moisture conditions areessential, secondary factors which relate to the specific resin usedinfluence which temperatures, within the range established by moistureconditions, are preferred. Thus, the composition of the resin, itsfilm-forming characteristics, strength and tackiness and the like allinfluence the optimum operating temperature. Of a diversity of resinstested, all were well handled within the range of -190 F. and most ofthe resins provided their best results within the range of from -185 F.

While it is possible to apply the required heat from the opposite sideof the wet coated paper, the total quality level is not as good and thecoating coverage is not as good. Also, there is a somewhat greatertendency for the coated paper to stick to the turning rolls.

The fibrous cellulosic substrates to be coated in accordance with theinvention can be of wide variety depending upon the use for which theproduct is intended. Thus they can be uncoated, coated, or sized webs,of either bleached or unbleached stocks, and can vary in weight fromlightweight papers to heavier paperboards. For purposes of moisturedrainage it is necessary that they be of a somewhat porous character.For example, paper which has simply been tubsized with an unpigmentedstarch or protein composition for increased internal strength provides aparticularly suitable substrate for application of the coatings of thisinvention. T 00, substrates which have previously been coated, whetherthe adhesive therein is water absorptive or not, can be used toadvantage since the stock is partially filled in such that less coatingneed be applied by the process of the invention to achieve a visuallycontinuous print-receptive film. For magazine papers and the like whereweight is of particular concern for reasons of economics, acomparatively lightweight stock of 45 pounds per ream or less canreadily be coated by the method of this invention to provide a highlyuseful printing paper of minimum weight. It is to be understood that theterm fibrous cellulosic paper substrate is inclusive of those non-wovensubstrates which are wholly cellulosic.

and those containing, in addition, non-cellulosic fibers, Whethernatural or synthetic.

Because of the relative high cost of the resin binder and owing to thenature of the leveling process, it is desirable that low weights ofcoating be applied and, preferably, at high speeds. It has been foundthat there is little to be gained by the use of coating Weights inexcess of 9 pounds per ream dry and, for most purposes, 6.5 pounds perream or less will satisfactorily provide the desired surfacecharacteristics.

With respect to the thermoplastic resins which are used in theinvention, several of these have been referred to specificallyhereinbefore, but it is desired to point out that the resin selectedshould be softenable and rendered mobile and film-forming by the heatwhich is applied by the turning rolls under the conditions or moisturewhich have been specified. Thus, and in the preferred practice of theinvention, even resins such as polyvinyl alcohol and starch which aresometimes regarded as non-thermoplastic after they have once been cured,are regarded as thermoplastic herein since, when freshly deposited fromthe aqueous medium, the heat applied effects an extensive softening ofthe resin providing high mobility within the thin layer of coatingmaterial so that the shearing action of the turning rolls can effect auniform distribution of the coating material over the paper and anextensive disruption and orientation of the particles of filler,especially dispersible mineral filler. In this respect clay particlesare of particular concern since the shearing forces disrupt theseforming many clay platelets to properly cover the surface of the paperat minimum coating weight. 011 the other hand, heat softenable materialswhich develop a degree of resistance to heat and solvent can also beemployed in accordance with the invention since, at the time ofsubjection to heat and pressure at the nip between the turning rolls,these resins are heat softenable. Thus, formaldehyde-modified casein maybe used, the formaldehyde providing a degree of thermosetting characterto the coating. It is stressed that thermoplasticity is to be measuredin the presence of Water vapor since some resins which do not flow wellunder heat and pressure have been found to flow quite well in thepresence of water vapor.

Especially suitable finely divided solid fillers are the mineralpigments or fillers, for example, clay, calcium carbonate, titaniumdioxide, zinc oxide and the like. Mixtures of these can be employed withadvantage.

In general, the temperature of the finishing roll should be in the rangeof 140 to 350 F. and the nip pressure in the range of 1500' to 7500pounds per square inch. Particularly preferred, however, is the range of3300 to 6000 pounds per square inch since this provides the greatestlatitude in selecting the other process variables. Although nippressures as great as 10,000 p.s.i. and as low as 500 p.s.i. can betolerated, they require critical adjustments of other variables andhence are less satisfactory for maximum operating efficiency.

The speed of operation may vary considerably, speeds of 200 to a 1000 ormore lineal feet per minute being practicable. Normally, it is preferredto operate at speeds in excess of 400- lineal feet per minute, withspeed being maximized with respect to the sophistication of theavailable equipment.

The coating material may be readily applied to the paper substrate byany of the conventional on-machine or off-machine procedures. It ispreferred to use the coater in the nature of a docton'ng device sincethi permits relatively low coating weights to be applied withoutsubstantial penetration of the paper stock.

After the paper has been coated and heat applied to provide thenon-equilibrium moisture contents previously referred to to therebyprovide a sandwich type structure including a water-plasticized layer ofpaper adjacent the coating-paper interface, the coated paper is passedbetween the turning rolls. The coated side of the paper would normallybe pressed into contact with an internally heated metal roll, preferablyprovided with a finishing surface such as polished chromium or the like.The turning rolls are driven at the same lineal speed as the coatedsubstrate. The turning roll which is used as a backup is preferablyformed to have an outer layer of material such as pressed cotton, nylon,asbestos or rubber which, although relatively hard has some resiliencyin comparison with the finishing roll.

As will be understood, the turning rolls function to shear the coatedsurface of the paper and it is desired to point out that the inventionemphasizes these shearing forces and, by virtue of the superiorlevelness achieved, the shearing forces are uniformly distributed overthe surface of the paper.

It is preferred to employ a waxy lubricant in the coating composition inorder to maintain on the finishing roll a surface having a minimumtendency to adhere to the coating which is applied to the papersubstrate. In order that the finishing roll be able to pick up thelubricant from the coating composition, the lubricant should have asoftening point below the surface temperature of the finishing roll.Desirably, the melting point of the lubricant should be in the range ofto 250 F. with the range of to 200 F. being particularly preferred foreffective high speed operations. Advantageously the lubricants areemployed as aqueous emulsions of finely divided waxy material which, incomparison with the resin, is non-film forming. Particularly preferredmaterials are those of a polar character such as the high meltingsugarcane waxes and oxidized hydrocarbon waxes which appear to form anadherent physical or chemical bond with the finishing surface to providea more stable layer, usually visible, of lubricant.

For effective utilization of a lubricant, it should in generalconstitute about 0.5 to 15% by weight of the total solids content of thecoating.

Aside from the hereinbefore described ingredients of the aqueous coatingcompositions, namely the particulate filler, resin and lubricant, it isentirely practical and often desirable to include certain additives, forexample dyes, plasticizers, viscosity regulators, dispersants for thelubricant, etc.

The invention will be more fully understood from the accompanyingdrawings in which:

FIG. 1 is a diagrammatic view illustrating the process of the invention;and

FIG. 2 is a diagrammatic sectional view on an enlarged scale of theproduct produced in FIG. 1.

Referring more particularly to FIG. 1 which illustrates the steps of theinvention, a paper substrate 11 is conveyed from an unwind roll 10 pastguide rolls 12 and 13 to a coating device 14 which comprises anapplicator roll 15, turning in either direction as desired, dipping in apan 16 containing aqueous coating 17. A rubber back-up roll 18 is usedto insure intimate contact with a doctor 19 constituted by a A" rotatingrod to remove excess coating and return it to the pan 16.

The substrate 11 with its undersurface coated with a thin layer ofcoating is then passed via guide rolls 22 through a heating zoneconstituted by a radiant heating device 20 positioned beneath thesubstrate. The heater 20 may be electric or gas-fired, but it isdesirably formed in sections which can be turned on and ofl, as desiredso that, and especially in conjunction with accurate control of speed,the precise control of temperature required by the invention can beobtained. The reflector 21 is optional, but is helpful to minimize heatrequirements.

On leaving the heating zone, the coated substrate passes guide roll 22and then proceeds to the calender stack 23 consisting of hydraulicallyloaded rolls lying between king and queen rolls 24. On the way, surfacetemperature may be measured continuously or periodically by an infraredpyrometer 25 which is positioned about 20 inches down stream from theheating unit 20 and held in close proximity to the heated surface anddirectly facing the same so that the surface temperature can be directlyobserved without interference.

The surface temperature reading may be used to either automatically ormanually adjust the operation of the equipment. in order that thepartially dried coating possess the non-equilibrium moisture conditionsfound to be critical. One may adjust line speed, heating rate or coatingweight and automatic control would be used wherever economicallyjustified.

The coated and heated substrate in its non-equilibrium condition is thenpassed between the nip formed by the rolls 26 and 27 in the calender 23.In the preferred operation, several successive heat and pressure nipsare used, three being shown, fly rolls 28 being used to guide the paperbetween the successive nips.

Finally, the finished paper is led to a wind-up roll 29.

FIG. 2 shows a cross-section of the product which is produced inaccordance with the invention. This product is constituted by acellulosic substrate 30 and a thin layer of coating material 31. In theusual situation, the weight of coating material is insufficient to fillthe surface volume of the paper substrate prior to treatment inaccordance with the invention, e.g., as it leaves the unwind roll 10.However, and in the product of the invention, the upper portion 32 ofthe substrate 30 is densified and is greatly leveled so that the smallamount of coating 31 is able to cover the same. This densification isillustrated in the diagrammatic sketch by the absence of voids 33 fromthe densified layer 32 despite the fact that such voids areindiscriminately distributed throughout the remainder of the substrate30. Not seen in FIG. 2 is the fact that the shearing forces developed inthe nip cause a considerable orientation of the mineral pigment portionof the coating so that, in addition to the levelness and uniformdistribution of coating material which is achieved, greater gloss isalso obtained and, as shown in the drawing, the coating is not driveninto surface voids in the paper as a result of the densification andleveling referred to.

It is desired to point out that the coated substrate is preferablypassed directly into the nip between the turning r0115 26 and 27.Apparently, passage directly into the nip tends to promote a greaterrelative slip within the underlying portion of the coating andplasticized portion of the substrate 32 with the consequence that thematerials thereof are exposed to a greater shearing force and, hence,greater leveling action. Such a shearing force is not as effectivelyprovided if the substrate wraps about a significant portion of thecircumference of either of the nip rolls prior to entry into the nipitself.

In FIG. 1 the roll 26 is an internally heated metal roll provided with afinishing surface and the roll 27 has a hard but resilient surface ofone or more of the materials mentioned earlier for a roll used as abackup roll. In the examples which follow, the heated rolls 26 have apolished chromium surface and the backup rolls 27 have a pressed cottonsurface.

Although the process has mainly been described with reference to anoff-machine application as would be conveniently performed on asupercalender, it will be apparent that for increased economicaladvantages the method is readily adapted to an on-machine operation. Instill other modificaitons it is entirely practical and, in fact,desirable for higher production to increase materially the width of thenip by employing nip rolls of a diameter beyond the ordinary size of 8to 20 inches. Alternatively, the finishing roll alone may be increasedin size.

The infrared pyrometer is used by holding it within about A; inch of theheated paper surface and is shielded to reduce interference from strayradiant sources. The device is calibrated against paper pressed againstan oil bath at known temperature within the range of from ISO-250 F. Thepyrometer used in the examples which follow is equipped with a Siemenshead identified by the nomenclature Ardonox 30-200 and is connected to aFoxboro recorder.

The invention is illustrated in the examples which follow:

Example 1 A t-ubsized paper stock weighing 62 pounds per ream is coatedwith 4 pounds per ream of an aqueous coating composition containing 40parts of precipitated calcium carbonate, 60 parts of coating clay, 4parts of polyvinyl alcohol and 1 part of wax emulsion at 46% by weightof total solids. The coated paper is dried to a surface temperature of190 F. in about 1.0 second and then passed through three nips using apressure of 800 pounds per lineal inch. The finishing rolls are chromeplated and heated with steam at 25 p.s.i.g. The sheet is moved throughthe calender rolls at a speed of 400 feet per minute to provide a glossreading of 65.

The above example indicates successful operation utiliz ing polyvinylalcohol but it will be understood that this resin requires very precisecontrol of temperature. Below 185 F., a tendency to deposit material onthe calender rolls is experienced. Utilizing temperatures in excess of195 F., the leveling and gloss fall off and become less accept-able.

Example 2 The same raw stock used in Example 1 is coated with an aqueouscoating composition containing 60 parts of coating clay, 40 parts ofprecipitated calcium carbonate, 15 parts of a low viscosity coatingstarch and 1 part of wax at 50% by weight of total solids. The coatedpaper is dried to a surface temperature of 172 F. in about 1.0 second.Again, the same number of nips heated in the same way and with the samepressure are used and the run is conducted at 400 feet per minute with acoating weight pick up of 4 pounds per ream. The product is entirelysatisfactory and has a gloss reading of approximately 60.

The starch used successfully in the above example provides a furtherillustration of resin selection which tends to require very precisecontrol of temperature for optimum results. Under the same conditionsreported, operation at a temperature of about F. encounteredconsiderable disruption of the film leading to heavy deposits on thecalender rolls. Operating at higher temperatures causes a significantloss in gloss, e.g., operating under the same conditions as reported butproducing a surface temperature of 189 F. lowers the gloss from thevalue of 60 reported to a value of 50. At still higher temperatures,e.g., employing a surface temperature of 204 F., the gloss is furtherimpaired to a gloss reading of 43 which approximates the best that canbe achieved utilizing conventional supercalendering.

Example 3 The same raw stock used in Example 1 is coated with an aqueouscoating composition containing 60 parts of coating clay, 40 parts ofprecipitated calcium carbonate, 20 parts carboxylated styrene-butadienecopolymer and 1 part of a wax emulsion, in a concentration of 58% totalsolids. The coated paper is dried to a surface temperature of F. Again,3 nips are used with 25 pounds steam pressure in each of the chromefinishing rolls. Four pounds of coating are picked up per realm and thepaper is treated at 400 feet per minute to provide a gloss reading of70.

Repeating the above example, but utilizing surface temperatures below160 F., serious filming on the calender rolls takes place so that theresults are not acceptable. Utilizing higher temperatures, e.g., a runutilizing a surface temperature of 183 F., provides substantially thesame results achieved at 170 F. Indeed, the results at 183 F. areslightly superior to those obtained at 170 F. illustrating the fact thatexcellent results are producible in the invention over a significantrange of temperature 9 so long as one remains within the critical rangeestablished hereinbefore. On the other hand, and when a surfacetemperature of 198 F. is employed, the gloss falls off to a value of 62,and at a surface temperature of 220 F., the gloss falls off to a valueof 52, again approximating the results of conventional supercalendering.

Example 4 The following example will illustrate the value of theinvention in achieving superior levelness and coating distribution atlow coating weight even when high gloss is not desired or obtainedthrough the use of mineral filler selected to provide a low glossfinish.

A tubsized paper stock weighing 62 pounds per ream is coated with 3pounds per ream of an aqueous coating composition containing 100 partsof ground calcium carbonate, 8 parts of ammonium caseinate, 1%formaldehyde, based on the weight of the ammonium caseinate, and 1 partof a wax emulsion at approximately 57% by weight of total solids. Thecoated paper is dried to a surface temperature of 193 F. in about 1.0second and then passed through three nips using a pressure of 800 poundsper lineal inch. The finishing rolls are chrome plated and heated withsteam at 25 p.s.i.g. The sheet is moved through the calender rolls at aspeed of 400 feet per minute. The final product has a gloss reading of20 and is characterized by a strikingly uniform appearance and anunusual resistance to marking, a defect normally present in paper coatedwith low gloss coatings.

In all of the foregoing examples, the gloss readings referred to are 15Bausch & Lornb Glossmeter readings. The invention is defined in theclaims which follow.

I claim:

1. A method for the continuous production of coated paper to obtain oneor more of superior levelness, improved coating distribution andsuperior gloss at lower coating weights comprising providing a dryfibrous cellulosic paper substrate having on one surface thereof, awater-wet layer of from 70-97% by weight of finely divided solidparticulate mineral-filler in intimate admixture with from 330% byweight of heat-softenable resinselected from the group consisting ofresins at least partially soluble in water and rubbery latex particles,rapidly heating said wet layer on said paper substrate until the exposedsurface of the coated paper has a surface temperature in the range offrom about 160 F. to about 195 F., so that vaporization of immediatelyavailable surface water is sufficient to break the continuity of thefilm of water at the surface without unduly reducing the availability ofwater near the surface of the coating whereby the water content of saidwet layer is substantially confined to that portion of said substrateimmediately underlying said layer, and passing said paper substrate inthe non-equilibrium moisture condition specified through the nip betweena pair of turning rolls, at least one of which has a heated finishingsurface, with said layer in contact with said finishing surface, saidturning rolls applying sufficient heat and pressure to soften said resinand to shear said layer to distribute the same and selectively densifythe portion of said substrate underlying said layer to provide thedesired coated paper.

2. A method as recited in claim 1 in which said layer is present in anamount up to 9 pounds dry per 3300 square feet of surface.

3. A method as recited in claim 1 in Which said substrate with the wetlayer thereon is heated for a period of less than 3 seconds.

4. A method as recited in claim 3 in which said wet layer is heatedradiantly from the side of said substrate surfaced with said layer.

5. A method as recited in claim 1 in which at least one of said rollshas a finishing surface heated to a temperature within the range of 190to 350 F., with said layer passing through said nip in contact with said'10 finishing surface, said turning rolls applying heat and a pressureat the nip of about 1500 to 7500 p.s.i.

6. A method as recited in claim 5 in which said finishing surface ispolished chromium having a molten deposit of waxy material thereon, saiddeposit being replenished from waxy material contained within the saidlayer in an amount of about 0.5 to 15 on a dry weight basis.

7. A method as recited in claim 1 in which said layer, on a dry weightbasis, contains about 0.5 to 15% by weight of waxy material.

8. A method as recited in claim 1 in which said layer contacts the saidfinishing surface substantially only within said nip and wherein thecoated paper is immediately Withdrawn from contact with said finishingroll upon emerging from said nip.

9. A method as recited in claim 1 in which said mineral filler includesclay.

10. A method as recited in claim 1 in which said exposed surface of thecoated paper is heated to a temperature in the range of from 170190 F.

11. A method as recited in claim 1 in which said heatsoftenable resincomprises polyvinyl alcohol.

12. A method as recited in claim 1 in which said rubbery latex comprisesa rubbery copolymer of styrene and butadiene.

13. A method for the continuous production of coated paper having afinished print-receptive surface comprising providing a dry fibrouscellulosic paper substrate having on one surface thereof, a water-wetlayer of from 7097% by weight of finely divided solid particulatemineral filler in intimate admixture with from 330% by weight ofheat-softenable resin selected from the group consisting of resins atleast partially soluble in water and rubbery latex particles, said wetlayer being deposited in an amount to provide from 1 to 9 pounds dry per3300 square feet of surface, rapidly heating said wet layer on saidpaper substrate to heat the exposed surface of the coated paper to asurface temperature in the range of from about F. to about 195 F. in aperiod of up to 3.0 seconds, so that vaporization of immediatelyavailable surface water is sufiicient to break the continuity of thefilm of water at the surface without unduly reducing the availability ofwater near the surface of the coating whereby the water content of saidwet layer is substantially confined to that portion of said substrateimmediately underlying said layer, and passing said paper substrate inthe non-equilibrium moisture condition specified through the nip betweena pair of turning rolls, at least one of which has a heated finishingsurface, with said layer in contact with said finishing surface, saidturning rolls applying sufiicient heat and pressure to soften said resinand to shear said layer to distribute the same and provide a glossedprint-receptive surface coating which is securely bonded to the saidsubstrate.

14. A method for the continuous production of coated paper having afinished print-receptive surface comprising the steps of:

(A) applying to a fibrous cellulosic paper substrate having a moisturecontent of from 2-13 and weighing at least 20 pounds per ream, a layerof aqueous coating containing from 7097% by weight of finely dividedsolid mineral filler in intimate admixture with from 330% by weight ofheat-softenable resin selected from the group consisting of resins atleast partially soluble in water and rubbery latex particles, saidcoating being applied in an amount to deposit from 1 to 9 pounds perream of dry coating Weight; (B) rapidly heating said wet layer on saidpaper substrate to heat the exposed surface of the coated paper to asurface temperature in the range of from 190 F. in a period of up to 1.5seconds, so that vaporization of immediately available surface water issufiicient to break the continuity of the film of water at the surfacewithout unduly reducing the availability of water near the surface ofthe coating whereby the water content of said wet layer is substantiallyconfined to that portion of said substrate immediately underlying saidlayer; and

(C) passing the resultant coated substrate in the nonequilibriummoisture condition specified through the nip between a pair of turningrolls, at least one of said rolls having a finished surface heated to atemperature Within the range of 190-350 F., said layer passing throughsaid nip in contact with said finishing surface, said turning rollsapplying a pressure at the nip of at least 500 psi.

15. A method as recited in claim 14 in which said aqueous coating has atotal solids content of from 35- 70% by weight.

16. Coated paper having a finished print-receptive surface comprising afibrous cellulosic substrate having bonded to at least one surfacethereof a visually continuous, print-receptive film comprising from7097% by weight of finely divided solid particulate mineral filleradhesively secured by from 3-30% by weight of heatsoftenable resinselected from the group consisting of resins at least partially solublein water and rubbery latex particles bonding the particles of saidfiller together into a film, said resin constituting the essential agentsecuring said film to said substrate, and said coated paper beingpermanently pressure deformed with the permanent deformation of thesubstrate being largely confined to the portion thereof underlying saidfilm to provide maximum leveling with minimum overall compression. I

17. The product of claim 16 in which said film is present in an amountup to 6.5 pounds per 3300 square feet.

18. A method for the continuous production of coated paper to obtain oneor more of superior levelness, improved coating distribution andsuperior gloss at lower coating weights comprising providing a dryfibrous cellulosic paper substrate having on one surface thereof, awater-wet layer of from 70-97% by weight of finely divided solidparticulate mineral filler in intimate admixture with from 330% byweight of casein, rapidly heating said wet layer on said paper substrateuntil the exposed surface of the coated paper has a surface temperaturein the range of from about 160 F. to about 195 F., so that vaporizationof immediately available surface Water is sufiicient to break thecontinuity of the film of water at the surface without unduly reducingthe availability of water near the surface of the coating whereby thewater content of said wet layer is substantially confined to thatportion of said substrate immediately underlying said layer, and passingsaid paper substrate in the non-equilibrium moisture condition specifiedthrough the nip between a pair of turning rolls, at least one of whichhas a heated finishing surface, with said layer in contact with saidfinishing surface, said turning rolls applying sufiicient heat andpressure to soften said casein and to shear said layer to distribute thesame and selectively densify the portion of said substrate underlyingsaid layer to provide the desired coated paper.

19. A method for the continuous production of coated paper to obtain oneor more of superior levelness, improved coating distribution andsuperior gloss at lower coating weights comprising providing a dryfibrous cellulosic paper substrate having on one surface thereof, aWater-wet layer of from 97% by weight of finely divided solidparticulate mineral filler in intimate admixture with from 330% byweight of starch, rapidly heating said wet layer on said paper substrateuntil the exposed surface of the coated paper has a surface temperaturein the range of from about F. to about F., so that vaporization ofimmediately available surface water is sufiicient to break thecontinuity of the film of water at the surface without unduly reducingthe availability of water near the surface of the coating whereby thewater content of said wet layer is substantially confined to thatportion of said substrate immediately underlying said layer, and passingsaid paper substrate in the nonequilibrium moisture condition specifiedthrough the nip between a pair of turning rolls, at least one of whichhas a heated finishing surface, said turning rolls applying sufficientheat and pressure to soften said starch and to shear said layer todistribute the same and selectively density the portion of saidsubstrate underlying said layer to provide the desired coated paper.

References Cited UNITED STATES PATENTS 2,321,938 6/1943 Quinn l17602,346,624 4/1944 Straus l17--64 2,554,663 5/1951 Cowgill 11765.22,759,847 8/1956 Frost et al. 1l764 2,826,827 3/1958 MetZ 11764 X2,919,205 12/1959 Hart l17-64 3,028,258 4/1962 Rice 117-64 WILLIAM D.MARTIN, Primary Examiner.

H. W. MYLIUS, M. LUSIGNAN, Examiners.

1. A METHOD FOR THE CONTINUOUS PRODUCTION OF COATED PAPER TO OBTAIN ONEOR MORE OF SUPERIOR LEVELNESS, IMPROVED COATING DISTRUBITION ANDSUPERIOR GLOSS AT LOWER COATING WEIGHTS COMPRISING PROVIDING A DRYFIBROUS CELLULOSIC PAPER SUBSTRATE HAVING ON ONE SURFACE THEREOF, AWATER-WET LAYER OF FROM 70-97% BY WEIGHT OF FINELY DIVIDED SOLIDPARTICULATE MINERAL FILLER IN INTIMATE ADMIXTURE WITH FROM 3-30% BYWEIGHT OF HEAT-SOFTENABLE RESIN SELECTED FROM THE GROUP CONSISTING OFRESINS AT LEAST PARTIALLY SOLUBLE IN WATER AND RUBBERY LATEX PARTICLES,RAPIDLY HEATING SAID WET LAYER ON SAID PAPER SUBSTRATE UNTIL THE EXPOSEDSURFACE OF THE COATED PAPER HAS A SURFACE TEMPERATURE IN THE RANGE OFFROM ABOUT 160*F. TO ABOUT 195*F., SO THAT VAPORIZATION OF IMMEDIATELYAVAILABLE SURFACE WATER IS SUFFICIENT TO BREAK THE CONTINUITY OF THEFILM OF WATER AT THE SURFACE WITHOUT UNDULY REDUCING THE AVAILABILITY OFWATER NEAR THE SURFACE OF THE COATING WHEREBY THE WATER CONTENT OF SAIDWET LAYER IS SUBSTANTIALLY CONFINED TO THAT PORTION OF SAID SUBSTRATEIMMEDIATELY UNDERLYING SAID LAYER, AND PASSING SAID PAPER SUBSTRATE INTHE NON-EQUILIBRIUM MOISTURE CONDITION SPECIFIED THROUGH THE NIP BETWEENA PAIR OF TURNING ROLLS, AT LEAST ONE OF WHICH HAS A HEATED FINISHINGSURFACE, WITH SAID LAYER INCONTACE WITH SAID FINISHING SURFACE, SAIDTURNING ROLLS APPLYING SUFFICIENT HEAT AND PRESSURE TO SOFTEN SAID RESINAND TO SHEAR SAID LAYER TO DISTRIBUTE THE SAME AND SELECTIVELY DENSIFYTHE PORTION OF SAID SUBSTRATE UNDERLYING SAID LAYER TO PROVIDE THEDESIRED COATED PAPER.